It should be tanked facing away from the raid while everyone stacks up onto the back of Overseer. Anyone not standing within 10 yards will be hit by Devastating Leap which does damage and stuns all nearby players. He also summons a lightning ball that will chase the targeted member and do damage every 0.5 sec.

Frontal cone attack (tank away from raid).

Stack up on his back to avoid damage and stun.

Summons lightning ball that will chase member and do AoE damage to nearby players (stand ground and heal through?)